(1) Technical Field
This invention relates to electronic circuits, and more particularly to signal attenuator circuits.
(2) Background
A signal attenuator is an electronic device that reduces the power of a signal without appreciably distorting its waveform, and is frequently used to lower voltage, dissipate power, and/or improve impedance matching. An attenuator is functionally the opposite of an amplifier (although the two work by different methods)—while an amplifier provides gain, an attenuator provides loss (or, equivalently, a gain less than one).
Attenuators may be passive devices made from simple voltage divider networks. However, for radio frequency (RF) applications, attenuators more typically comprise a small network of passive (and, optionally, active) devices. Classic examples of single-valued RF attenuators are two-port bridged-T type, pi-type, T-type, and L-pad type attenuators.
It is known to serially connect multiple single-valued RF attenuators of various types to provide for a selectable amount of attenuation in a circuit. Sometimes known as a digital step attenuators (DSAs), such attenuators are frequently used with RF systems such as transceivers for broadcast radio, cellular telephones, RF-based data networks (e.g., WiFi, Bluetooth), and RF test instruments.
A conventional DSA consists of a series cascade of switchable single-valued RF attenuator cells, with binary weighted attenuation values; a more advanced type of DSA includes both binary and thermometer weighted attenuator cells. Examples of that latter type of DSAs are shown in U.S. patent application Ser. No. 14/084,439 referenced above.
A switchable attenuator cell is designed to have two selectable states: (1) an attenuation state, and (2) a bypass or “through” state. The bypass state is normally provided by a switch connected in parallel with the input and output ports of an attenuator network (e.g., two-port bridged-T type, pi-type, T-type, and L-pad type attenuators). The switch is typically a field effect transistor (FET), and is commonly a MOSFET.
To withstand high input power in conventional DSAs, a number of bypass switches (e.g., FETs) may be series connected or “stacked” within an attenuator cell to withstand the increased applied voltage and power levels. However, an increase in the series stack size leads to increases in insertion loss (IL), parasitic capacitances and inductances, integrated circuit (IC) die area, and power dissipation.
Conventional thermometer or mixed binary/thermometer weighted DSAs can even make the problem worse compared to binary weighted DSAs because of the increased number of attenuator cells needed for thermometer weighting. If each thermometer attenuator cell is designed to withstand high power, the total series stack size becomes very large, thereby exacerbating the problems noted above.
The present invention addresses the above limitations of existing DSAs, and includes a number of embodiments that have reduced IL, parasitic capacitances and inductances, IC die area, and power dissipation compared to conventional DSA circuits.